The UK and the US have agreed to amend an “anomaly” that allowed Harry Dunn death suspect Anne Sacoolas to claim diplomatic immunity.
A court hearing last month heard the “secret agreement” permitted Mrs Sacoolas to return to America after the 19-year-old’s death last August.
He was killed in a crash near RAF Croughton, Northamptonshire.
Speaking to the Press Association (PA), Harry’s mother Charlotte Charles said the change was a “huge step forward”.
She said the family’s campaign for Mrs Sacoolas – the wife of a US intelligence official at RAF Croughton – to return to the UK would continue.